I use Fraps and it works wonders, records in a lossless format so you get the same quality as you get when playing - quite lovely really! Though it might take some getting used to if you're not used to playing at 30FPS (unless you want to record it at a set 60fps which might be a bit overkill on your HD).

But yes as been stated, have a beefy HD ready and a good rig in general as it is very taxing on the system. This is not a job for a laptop if you catch my drift!

Word of advice when using fraps: It's codec isn't brilliant, so if you plan on doing anything more than compressing the footage it is worth encoding the footage to another lossless format (Lagarith is a good choice here, you should be able to reduce file-size a little without any quality loss too as an added benefit).